1. Field of the Invention
The present invention relates to an optical recording medium, particularly, to a holographic recording medium, and a method of manufacturing the same.
2. Description of the Related Art
A holographic memory that permits recording information in the form of a hologram is capable of a large capacity recording and, thus, attracts attention as a recording medium of the next generation. As the photosensitive composition for recording the hologram, known is a composition which contains, for example, a radically polymerizable monomer, a thermoplastic binder resin, a photo radical polymerization initiator, and a sensitizing pigment as the main components. The photosensitive composition for recording the hologram is formed into a film and, then, an interference light exposure is performed, thereby recording the information.
In the portion irradiated with light of a high intensity, the polymerization reaction of the radically polymerizable monomer proceeds, and the radically polymerizable monomer is diffused from the portion irradiated with light of a low intensity toward the portion irradiated with light of a high intensity so as to bring about the concentration gradient. In other words, a difference in density is generated in the radically polymerizable monomer in accordance with the intensity of the interference light so as to bring about a difference in the refractive index.
Proposed in recent years are a recording medium prepared by dispersing radically polymerizable monomers in an epoxy matrix. The recording medium is disclosed in “Epoxy-Photopolymer Composites: Thick Recording Media for Holographic Data storage (T. J. Trentler, J. E Boid and V. L. Colvin)”; Proceedings of SPIE, 2001, Vol. 4296, pp259-266, for example. In preparing the recording medium, N-vinylcarbazole and N-vinylpyrrolidone are used as the radically polymerizable monomers. Specifically, N-vinylcarbazole, which is in the form of a solid, is dissolved in N-vinylpyrrolidone, which is in the form of a liquid under room temperature, and the resultant solution is mixed with an epoxy compound. It is said that the equimolar amount is the upper limit of N-vinylcarbazole that can be dissolved in N-vinylpyrrolidone, i.e., N-vinylcarbazole cannot be dissolved in N-vinylpyrrolidone in a molar amount larger than the molar amount of N-vinylpyrrolidone. In terms of the weight of N-vinylcarbazole that can be dissolved in N-vinylpyrrolidone, the upper limit of the weight ratio of N-vinylcarbazole (VC) to the sum of N-vinylcarbazole (VC) and N-vinylpyrrolidone (VP), i.e., VC/(VC+VP), is 0.634.